The invention relates to a process and apparatus for operating a system for water vapor reforming of methanol.
For operating such systems, it is known to vary the amount of the processed water vapor/methanol mixture which is introduced into the reforming reactor as a function of the load condition and the load change of the system. In German Patent Documents DE 1 949 184 and DE 21 57 722 C2 (in which methane is used instead of methanol), for example, a jet pump is provided for this purpose, and the vapor flow is adjusted as a function of the loading condition of the system. Thus, during a load change, the methane inflow also changes corresponding to the jet pump characteristics.
U.S. Pat. No. 5,401,589 discloses a process for operating a fuel cell system in a motor vehicle by means of a reformer system which is connected in front of the fuel cells and uses water vapor reforming of methanol for the purpose of obtaining the hydrogen required for the fuel cells. In order to be able to react to load changes faster than by merely changing the amount of the water vapor/methanol mixture introduced into the reforming reactor, a buffer battery and an oxygen reservoir are provided. In the case of an increased load requirement, additional oxygen is taken from the oxygen reservoir for introduction into the fuel cell system. It is also known from this as well as other documents that, when the water vapor concentration in the water vapor/methanol gas mixture is too low, during its reforming an increased CO-concentration occurs in the reformate. This is undesirable, particularly when the reformate is used as a fuel in so-called PEM fuel cells because these are poisoned by carbon monoxide. Normally, the water vapor/methanol mixing ratio is therefore maintained within a range of between one and ten.
During the operation of systems for the water vapor reforming of methanol, it is observed that in the case of a load change (that is, a change of the quantity of water vapor/methanol mixture introduced into the reforming reactor), a change of the water vapor/methanol mixture ratio occurs, due to a momentary change of the evaporation conditions in the system. This, in turn, leads to fluctuations in the CO-concentration of the reformate, which can cause undesirably high CO-concentrations at times, which must be removed in a purification process which follows.
The object of the invention is to provide a process of the initially mentioned type which avoids an occurrence of undesirable CO-concentrations in the reformate, during dynamic load changes as well as in static operation.
This and other objects and advantages are achieved by the process according to the invention, in which the water vapor/methanol mixing ratio of the water vapor/methanol mixture prepared in the mixture preparation step and introduced into the reforming reactor, is adjusted as a function of the time variation of the load condition, so that a CO-concentration is obtained in the reformate which is constant over the whole load range. As required, such adjustment of the water vapor/methanol mixing ratio can take place by a pure control, without an absolute requirement of feedback regulation. Keeping the CO-concentration constant in the reformate prevents undesirable CO peak values and permits a simple design of a gas purification step optionally connected behind the reforming reactor, which removes undesirable carbon monoxide. The process therefore advantageously adapts the method of operation to the loading of the system, avoiding increased CO-concentrations in the reformate.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.